This invention relates generally to magnetic resonance spectroscopy, and more particularly, the invention relates to volume spectroscopy in which water baseline artifact signal is reduced.
Volume localized magnetic resonance spectroscopy has become a useful and routine clinical tool especially for the detection of abnormalities which lead to diffused chemical changes in the brain. Several techniques are known for directly exciting spins in a volume of interest and achieving three-dimensional selection including use of stimulated echoes and the use of Carr-Purcell echoes. These techniques obtain a localized spectrum in a single scan. For example, point resolved spectroscopy (PRESS, see U.S. Pat. No. 4,480,228) uses a three pulse sequence with each pulse being frequency selective.
Many important clinical applications of proton magnetic resonance spectroscopy are based on the use of a restricted volume of excitation. Typically, the volume excitation is achieved using PRESS, which takes advantage of three orthogonal slices in the form of a double spin echo to select a specific region of interest.
There is a strong desire for absolute quantitation of localized proton spectroscopy of brain. This is an imposing task complicated by severe overlap of signals, baseline artifacts from spurious side bands of residual water, unknown T.sub.2 losses, and in some cases partial volume issues. Although there are a number of independent approaches to eliminating artifacts, simplifying or editing spectra, T.sub.2 estimation, and partial volume correction, there is a great deal of error and inefficiency in combining these methods to extract all of the desired information.
Ryner et al. "LOCALIZED 2D J-RESOLVED .sup.1 H MR SPECTROSCOPY: STRONG COUPLING EFFECTS IN VITRO AND IN VIVO," Magnetic Resonance Imaging, Vol. 13, No. 6, pp 853-869, 1995, disclose a two-dimensional (2D) J-resolved MR spectroscopy sequence (2D J-PRESS), fully localized in three dimensions, implemented on a whole-body MR scanner. As described by Ryner et al., transverse magnetization created by a 90.degree. RF pulse can be refocused with a 180.degree. RF pulse to create a spin-echo after an evolution during the echo time TE. Linear interactions like a chemical shift, static field inhomogeneity, resonance offset, etc., are averaged out during TE. The decay of the transverse magnetization is solely due to T.sub.2 relaxation, neglecting effects due to spin diffusion and chemical exchange. Bilinear interactions, namely J-coupling, are unaffected by the refocusing pulse, leading to J-modulation of the magnetization during TE.
The 2D J-resolved sequence was first demonstrated over two decades ago, using a conventional spin-echo pulse scheme on a high-resolution NMR spectrometer. The processing as a 2D J-resolved data set includes Fourier transform along two axes (t1, t2) with chemical shift and J-coupling frequencies present along the resulting F2 axis and J-coupling frequencies separated along the resulting F1 axis.